Simulation Optimizes Design of Selective Catalytic Reduction Systems to Meet Emissions Challenges

The United States Environmental Protection Agency (EPA) and the European Union (Euro 6 standards) have implemented stringent diesel emissions standards for 2010 and 2013 respectively that force a dramatic reduction in discharges of particulate matter and nitrogen oxide (NOx). Current state-of-the-art in-cylinder solutions have fallen far short of achieving these limits. Selective catalytic reduction (SCR), which converts NOx with the aid of catalyst to nitrogen and water, is one exhaust aftertreatment method that is being considered to meet emissions requirements. SCR system designers face several technical challenges, such as avoiding ammonia slip, urea crystallization, low temperature deposits and other potential pitfalls. Simulation can help to develop a deep understanding of these technical challenges and issues, identify root causes and help develop better designs to overcome them. The latest generation of computer simulation tools partially automates the creation of the analytical model; incorporates fluid flow, chemical reactions, thermal and structural analysis to gain a complete understanding of SCR performance; and iterates to an optimized design with minimal user intervention.

Tougher diesel emissions standards

The 2010 EPA diesel emissions standards reduced emissions for on-highway engines 90% from 2004 levels, to 0.2 gram per brake horsepower hour (g/bhp-hr) for NOx and 0.01 g/bhp-hr for particulate. For off-highway engines, NOx must be reduced to 1.5 g/HP-hr, and the particulate standard is reduced to 0.01 g/HP-hr PM. The Euro 6 standards require NOx and particulate emissions for diesel on-highway heavy duty engines to be below 0.4 (g/kW-hr) and 0.01 (g/kW-hr) respectively. Substantial improvements were achieved starting in 1996 with the introduction of ultralow sulfur diesel fuel, which emits only 11 ppm of sulfur, a 97% reduction compared to the previous diesel fuel generation. Further improvements followed with the adoption of advanced combustion technology, which involves fine-tuning injection parameters such as injection pressure, injection timing, shape of injection profile, etc. Cooled exhaust gas recirculation (EGR) systems have been used since 2002 by major engine manufacturers such as Cummins, Detroit Diesel, International, Mack and Volvo to reduce NOx emissions from diesel truck engines. These systems divert from 5 to 30% of an engine’s exhaust stream through an air-to-water cooler, then back into the combustion chambers, where the cooled gases reduce peak temperatures and, thus, retard NOx formation. They have been instrumental in meeting pre-2010 standards.

However, the new regulations have forced engine manufacturers to look beyond reducing emissions at the source and focus on treating the exhaust gases after they leave the engine. Aftertreatment techniques for NOx reduction include SCR, NOx adsorbers and lean-NOx catalysts. SCR is the most popular NOx aftertreatment technology. Here’s how it works: Aqueous urea is injected just upstream of the catalyst. Urea decomposes into ammonia and isocyanic acid. Ammonia and isocyanic acid react with NOx over the catalyst, producing nitrogen and water. SCR provides high NOx conversion efficiency and is less sensitive to sulfur. Typically, for a simple open-loop system, the conversion efficiency is 65%, which can be increased up to 95% with a sophisticated closed-loop system. Further, higher NOx conversion allows tuning the engine combustion system for better fuel consumption. Engines with SCR technology provide up to 5% better fuel economy compared to 2010 non-SCR engines.

SCR design challenges